1. Field of the Invention
The present invention relates to an optical servo control apparatus and method for use with media having information carrying regions and servo control regions of different reflectivity and more particularly to such an apparatus and method which utilizes an unfocused light source, a photodetector having at least two specially shaped photodetection cells and media having servo control regions comprised of equally spaced grooves or other contrasting areas on the surface of the medium. An electronic circuit processes the output signal from the photodetector so as to permit electronic positioning of a data head over the information carrying region, to permit positioning of the data head when the information carrying region comprises more than one data track per servo track and to permit the data head to change tracks without leaving the position loop.
2. Description of the Prior Art
The track density of magnetic storage disks of conventional magnetic floppy disk drives is approximately forty-eight to one hundred thirty-five tracks per inch. In contrast, optical disk drives are capable of achieving track densities in excess of 15,000 tracks per inch. These higher track densitites are achieved through the use of closed loop optical servos that allow the read/write head to follow data track eccentricities caused by defects in the medium and by disturbances from outside forces.
Various techniques have been reported for using optical means for acquiring servo information contained on a magnetic recording medium. For example, K. Ahn, et al, in U.S. Pat. No. 4,633,451 for "Optical Servo For Magnetic Disks", disclose the use of a laser diode to read servo information in the form of a plurality of spots contained in an optical layer positioned above a magnetic recording layer.
T. DiStefano, et al, in U.S. Pat. No. 4,570,191 for "Optical Sensor for Servo Position Control", disclose a servo sensor comprising a light source and a light detector, axially aligned and contained on a single semiconductor chip.
M. Johnson, in U.S. Pat. No. 4,558,383 for "Information Storage Disk Transducer Position Control System Using a Prerecorded Servo Pattern Requiring no Alignment with the Storage Disk", discloses a servo apparatus having a sensor for detecting a pattern of spots on a surface of an information storage medium. The spots comprise a dense array of substantially translation invariant marks and separate information recording tracks are detected by measuring the rate at which the spots are detected by the sensor.
J. Cocke, et al, in U.S. Pat. No. 4,587,579 for "System for Position Detection on a Rotating Disk", disclose a servo control system comprising a detector for reading a plurality of spiral radial-position encoding patterns on a medium.
A. S. Hoagland in "Optical Servo of Magnetic Recording", IBM Technical Disclosure Bulletin, Vol. 20(10), page 4108 (March 1978), suggests a system for achieving optical servo control where a flexible disk medium includes a plurality of optical servo tracks positioned underneath a magnetic layer.
N. Koshino and S. Ogawa in "Optical Method of the Head Positioning in Magnetic Disk Systems", preprint from IEEE Transactions on Magnetics (1980), disclose an optical head for achieving servo control which is mounted on the head arm and which includes an LED light source and three optical fibers for delivering light to a medium. The medium comprises a plurality of circular optical tracks, dyed black, and located underneath a magnetic film.
Related development has occurred in the compact disk (CD) industry where laser detection systems are used to read the very closely spaced laser etched tracks. In a three-beam tracking technique, a six element photodetector cell is utilized. This photodetector has six light sensitive areas, or cells, arranged so that there are four centrally located square or triangular cells flanked by two outer rectangular shaped cells. Typically, the center cells are used for the focus servo function while the two outside cells are used for the tracking servo function. In a far field tracking technique, a photodetector having only four cells is utilized. Both of these techniques depend on the well behaved characteristics of a focused beam of coherent light generated by a laser light source for their proper operation. Additionally, to the inventors' knowledge, all reported optical tracking techniques utilize media having no more than one data track per servo track. Furthermore, all reported serving techniques used in storage devices are unable to move continuously between data and servo tracks while retaining the ability to stop at any point between the data and servo tracks without leaving the position loop.